1. Field of the Invention
The present invention relates to scanning apparatus, in particular to scanning apparatus, in particular to a scratch-suppressing scanning apparatus.
2. Background of the Invention
Scanning apparatus for producing an electrical representation of an image on one face of a transparent image carrier are known. Such apparatus comprises a light source, a support surface for the image, which is movable in one direction, optical means for directing light from the light source onto the image carrier to illuminate a scan line on the image, and an objective lens for projecting a scan line of the image on light-sensitive detection means.
A main problem encountered in scanners is caused by scratches in the transparency, non in particular in its rear side to be projected. The appearance of a scratch results in a white or dark line in the resulting picture, depending upon whether the transparency is a negative or a positive. Each facet of a scratch acts like a prism that refracts light passing through the film. A ray of light that would normally pass through the film and thus illuminate the image on the front side of the film is refracted in a direction deviating from the normal one. Much of the light refracted by scratches in this manner escapes from the original path of the scanning apparatus and is not detected by the photosensors in the case of film scanners. Thus, a streaklike area of the final picture looks as if it would not have been exposed, which means a black line disturbing the image the case of a positive transparency.
One arrangement for reducing the effect of scratches is disclosed in U.S. Pat. No. 4,395,108 of Eastman Kodak Cy. this document teaches scratch-suppressing projection apparatus, which includes an objective lens defining first and second congregate planes and an integrating bar disposed between a condenser lens behind the transparency to be scanned, and light-sensitive means, e.g. photomultipliers. Disadvantages of this apparatus are the limited light output of the illumination source, viz. a flying spot scanner, and the limitation to transparencies of reduced format, such as 135 size.
Another useful arrangement for suppressing scratches is disclosed in U.S. Pat. No. 4,868,383 of Eastman Kodak Cy. This document deals with a linear light source for a film scanner including an elongated cylindrical integrating cavity having diffusely reflective walls and an output slit parallel to the longitudinal axis of the cavity. The diffuse illuminating light is favourable for avoiding the reproduction of scratches, but it has the disadvantage of a reduced yield so that the light output is correspondingly limited.
Still another scanning system is disclosed in U.S. Pat. No. 3,988,537 of A-G Cooley. The illuminating optical system includes a fluorescent lamp, and first and second cylindrical lenses and an illuminating aperture plate. A scanning optical axis is preferably displaced from the illuminating optical axis. A disadvantage of cylindrical lenses is their rather long focal length, and also the very non-uniform light- distribution over their projection angle so that this system is not suited for reducing the effect of scratches.